The potential for using cellular proteins for biosensing and drug discovery applications has long been appreciated. However there are many technical challenges to overcome in developing this technology to fully realise the potential. There is a wealth of literature on using fluorescent and optical approaches, but the focus of this document is on the measurement of electrical signals to recognise analytes in biosensing.
In one type of technique, a layer of amphiphilic molecules may be used as the layer separating two volumes of aqueous solution. The layer resists the flow of current between the volumes. A membrane protein is inserted into the layer to selectively allow the passage of ions across the layer, which is recorded as an electrical signal detected by electrodes in the two volumes of aqueous solution. The presence of a target analyte modulates the flow of ions and is detected by observing the resultant variations in the electrical signal. Such techniques therefore allow the layer to be used as a biosensor to detect the analyte. The layer is an essential component of the single molecule biosensor presented and its purpose is two-fold. Firstly the layer provides a platform for the protein which acts as a sensing element. Secondly the layer isolates the flow of ions between the volumes, the electrical resistance of the layer ensuring that the dominant contribution of ionic flow in the system is through the membrane protein of interest, with negligible flow through the bilayer, thus allowing detection of single protein channels.
A specific application is stochastic sensing, where the number of membrane proteins is kept small, typically between 1 and 100, so that the behaviour of a single protein molecule can be monitored. This method gives information on each specific molecular interaction and hence gives richer information than a bulk measurement. However, due to the small currents involved, typically a few pA, requirements of this approach are a very high resistance seal, typically at least 1 GΩ and for some applications one or two orders of magnitude higher, and sufficient electrical sensitivity to measure the currents. While the requirements for stochastic sensing have been met in the laboratory, the conditions and expertise required limit its use. In addition, the laboratory methods are laborious and time-consuming and are not easily scalable to high-density arrays, which are desirable for any commercial biosensor. Furthermore, the fragility of single bilayer membranes means that anti-vibration tables are often employed in the laboratory.
By way of background, existing techniques for forming layers of amphiphilic molecules such as lipid bilayers will be reviewed.
Several methods for forming planar artificial lipid bilayers are known in the art, most notably including folded bilayer formation (e.g. Montal & Mueller method), tip-dipping, painting, patch clamping, and water-in-oil droplet interfaces.
At present, the bulk of routine single ion channel characterisation in research labs is performed using folded bilayers, painted bilayers or tip-dip methods. These methods are used either for the ease of bilayer formation, or for the high resistive seals that can be formed (e.g. 10-100 GΩ). Tip-dip bilayers and bilayers from patch-clamping of giant unilamellar liposomes are also studied as they can be formed in a solvent free manner, which is thought to be important for the activity of some protein channels. The method of Montal & Mueller (Proc. Natl. Acad. Sci. USA. (1972), 69, 3561-3566) is popular as a cost-effective and relatively straightforward method of forming good quality folded lipid bilayers suitable for protein pore insertion, in which a lipid monolayer is carried on the water/air interface past either side of an aperture in a membrane which is perpendicular to that interface. Typically, the lipid is added to the surface of the aqueous electrolyte solution by first dissolving it in an organic solvent, a drop of which is then allowed to evaporate on the surface of the aqueous solution on either side of the aperture. Once the organic solvent has been evaporated, the solution/air interfaces are physically moved up and down past either side of the aperture until a bilayer is formed. The technique requires the presence of a hydrophobic oil applied as a pre-treatment coating to the aperture surface. The primary function of the hydrophobic oil is to form an annulus region between the bilayer and the aperture film where the lipid monolayers must come together over a distance typically between 1 and 25 μm.
Tip-dipping bilayer formation entails touching the aperture surface (e.g. a pipette tip) onto the surface of a test solution that is carrying a monolayer of lipid. Again the lipid monolayer is first generated at the solution/air interface by evaporating a drop of lipid dissolved in organic solvent applied to the solution surface. The bilayer is then formed by mechanical actuation to move the aperture into/out of the solution surface.
For painted bilayers, the drop of lipid dissolved in organic solvent is applied directly to the aperture, which is submerged in the aqueous test solution. The lipid solution is spread thinly over the aperture using a paint brush or equivalent. Thinning of the solvent results in formation of a lipid bilayer, however, complete removal of the solvent from the bilayer is difficult and consequently the bilayer formed is less stable and more noise prone during measurement.
Patch-clamping is commonly used in the study of biological cell membranes, whereby the cell membrane is clamped to the end of a pipette by suction and a patch of the membrane becomes attached over the aperture. The method has been adapted for artificial bilayer studies by clamping liposomes which then burst to leave a lipid bilayer sealing over the aperture of the pipette. This requires stable giant unilamellar liposomes and the fabrication of small apertures in glass surfaced materials.
Water-in-oil droplet interfaces are a more recent disclosure in which two aqueous samples are submerged in a reservoir of hydrocarbon oil containing lipid. The lipid accumulates in a monolayer at the oil/water interface such that when the two samples are brought into contact a bilayer is formed at the interface between them.
In any of these techniques, once the bilayer has been formed, the protein is then introduced to the bilayer either by random collision from the aqueous solution, by fusion of vehicles containing the protein, or by mechanically transporting it to the bilayer, for example on the end of a probe device such as an agar tipped rod.
There have been great efforts recently to increase the ease of bilayer formation using micro fabrication. Some techniques have attempted essentially to miniaturise standard systems for folded lipid bilayers. Other techniques include bilayer formation on solid substrates or directly on electrode surfaces, through either covalent attachment or physical adsorption.
A large proportion of the devices that are capable of performing stochastic sensing form a bilayer by using a variant of the folded lipid bilayers technique or the painted bilayer technique. To date most have concentrated either on novel methods of aperture formation or on utilising the emerging technologies in micro fabrication to miniaturise the device or to create a plurality of addressable sensors.
An example is Suzuki et al., “Planar lipid bilayer reconstitution with a micro-fluidic system”, Lab Chip, (4), 502-505, 2004. Herein, an aperture array is created by etching a silicon substrate, followed by a surface treatment to encourage the bilayer formation process, although the disclosed rate of successful bilayer formation is very low (two out of ten).
A more recent example is disclosed in Sandison, et al., “Air exposure technique for the formation of artificial lipid bilayers in microsystems”, Langmuir, (23), 8277-8284, 2007. Herein the device fabricated from poly(methylmethacrylate) contains two distinct aqueous chambers. Problems with the reproducibility of bilayer formation are attributed to the difficulty in removing the excess hydrophobic material from the aperture, and tackled by using a period of air exposure to aid the bilayer formation process to thin the pre-treatment.
The devices of both Sandison et al. and Suzuki et al. are both miniaturised versions of a standard painted bilayer technique with two distinct fluidic chambers separated by a septum containing an aperture across which the bilayer is formed, one chamber being filled before the other. This presents a number of difficulties for scaling up the system to a large number of individually addressable bilayers, as at least one of the aqueous chambers must be a distinct chamber with no electrical or ionic connectivity to any other chamber. Sandison et al. created a device with three fluid chambers, each with separate fluidics, an approach which would be difficult to scale to large numbers of bilayers. Suzuki et al. tried to address this problem by using a hydrophobic photoresist layer to create small aqueous chambers on top of the aperture containing substrate. In this case, it is difficult to control the flow of solution across the aperture containing interface and the use of small volumes exposed to air makes the apparatus susceptible to evaporation effects. In both cited examples, the need for the individual aqueous chambers for each bilayer means that a large sample volume must be used to fill all the chambers.
An example of biosensor device using a supported lipid bilayer is disclosed in U.S. Pat. No. 5,234,566. The device is capacitive. A gated ion channel responds to an analyte, the binding of this analyte causes a change in the gating behavior of the ion channel, and this is measured via the electrical response of the membrane capacitance. To support the lipid bilayer, there is used a monolayer of alkane-thiol molecules on a gold electrode, which provides a scaffold for a lipid monolayer to self-assemble onto. This monolayer can incorporate ion channels such as gramicidin which are used as the sensing element of the device. Variations on this method have been used to create a tethered lipid bilayer onto an electrode surface to incorporate other membrane proteins. However, the approach has a number of drawbacks, the first is that the small aqueous volume present under the lipid bilayer, typically of the order of 1 nm to 10 nm thick, does not contain enough ions to perform a direct current measurement for any useful period of time. This is an effect common to nearly all tethered bilayer systems on solid supports. For recordings of any meaningful duration, an alternating current measurement must be used to overcome the ionic depletion at the electrode, but that limits the sensitivity of the device.
An example of a biosensor device using a supported lipid bilayer is disclosed in Urisu et al., “Formation of high-resistance supported lipid bilayer on the surface of a silicon substrate with micro electrodes”, Nanomedicine, 2005, (1), 317-322. This device exploits the strong surface adhesion between phospholipid molecules and a SiO2 surface to form a supported bilayer. A silicon oxide surface is modified, using etching techniques common in silicon chip production, to expose small channels to an electrode surface. A bilayer is then formed on the silicon oxide surface, resulting in an electrical resistance of a few MΩ. In this system, the wells created by this process could not be individually addressed.
In both of the cited examples using a supported lipid bilayer, it is very difficult to form a high resistive seal using these methods. Although the resistance may be sufficient to observe a change arising from a large number of ion channels, single channel or stochastic measurements, which are inherently more sensitive, are incredibly challenging using this methodology.
There are a number of problems with the supported bilayer approach in these documents and in general, which makes this system unsuitable. The first problem lies with the resistance of the bilayer membrane which is typically about 100MΩ. While this may be suitable for examining protein behaviour at large protein concentrations, it is not sufficient for a high-fidelity assay based on single molecule sensing, typically requiring a resistance of at least 1 GΩ and for some applications one or two orders of magnitude higher. The second problem is the small volume of solution trapped in the short distance between the bilayer and the solid support, typically of the order of 1 nm. This small volume does not contain many ions, affecting the stability of the potential across the bilayer and limiting the duration of the recording.
A number of methods have been proposed to overcome the problems with solid supported bilayers. One option is to incorporate a chemical linkage between the bilayer and the surface, either a small polyethylene glycol layer is introduced (polymer cushioned bilayers), or the lipid is chemically modified to contain a small hydrophilic linkage and reacted with the surface providing a scaffold for vehicle deposition (tethered bilayers). While these methods have increased the ionic reservoir beneath the lipid bilayer, they are inconvenient to implement and have done little to decrease the current leakage across the bilayer.
The techniques used in the silicon chip industry provide an attractive technology for creating a large number of electrodes that could be used in biosensor applications. This approach is disclosed in the related applications U.S. Pat. No. 7,144,486 and U.S. Pat. No. 7,169,272. U.S. Pat. No. 7,144,486 discloses a method of fabricating a microelectrode device containing microcavities etched into layers of insulator material. The devices are said to have a wide range of electrochemical applications in which electrodes in the cavities measure electrical signals. It is stated that thin films may be suspended across the cavities. Several types of film are mentioned, including being a lipid bilayer. However this is merely a proposal and there is no disclosure of any technique for forming the lipid bilayer, nor any experimental report of this. Indeed the related application U.S. Pat. No. 7,169,272, which does report experimental formation of lipid bilayers in the same type of device, discloses the supported lipid bilayers being chemically attached directly on the electrodes. This uses similar techniques to those presented in Osman et al. cited above and suffers from the same drawbacks relating to the lack of a sufficiently high resistive seal for stochastic measurements and the lack of an ionic reservoir for recording ionic flow across the bilayer system.
To summarize, the technologies described above either present methods of bilayer formation which can not reproducibly achieve high resistance, or suffer from low ionic reservoirs and are not capable of high duration direct current measurements, or require a separate fluidic chamber for each array element, limiting the scale up of that device to a high-density array. It would be desirable to reduce these problems.